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Time-Dependent Spectrophotometric Study of the Interaction of Basic Dyes with Clays
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
198, 106–112 (1998)
CS975268
Time-Dependent Spectrophotometric Study of the Interaction of Basic Dyes with Clays III. Mixed Dye Aggregates on SWy-1 and Laponite Ana P. P. Cione, Miguel G. Neumann, 1 and Fergus Gessner 1 Instituto de QuıB mica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Caixa Postal 780, 13560-970 Sa˜o Carlos SP, Brazil Received July 7, 1997; accepted October 24, 1997
The interactions of a pair of different cationic dyes (methylene blue and acridine orange) in the presence of dilute aqueous suspensions of the montmorillonite SWy-1 and Laponite clays were studied using spectrophotometric techniques. A standardized procedure was developed to prepare the samples and perform the spectrophotometric measurements of the clay-dye suspensions. The observed nonadditivity of the absorption spectra of the mixture compared with those of the individual dyes is attributed to the formation of mixed aggregates on the clay particles. Significant spectral variations with time were observed: for suspensions containing the montmorillonite SWy-1 they were mainly due to the reorganization of the dye molecules on the clay particles. For systems containing Laponite, the spectral changes were ascribed to processes of approximation-association of the clay particles. At longer times (24 h) the association processes prevail, even for the SWy-1 suspensions. After sufficient time, both the SWy-1 and the Laponite suspensions showed similar spectra, indicating that the processes occurring in both systems lead to similar configurations. Schemes are proposed to describe the temporal variations occurring in these systems. q 1998 Academic Press Key Words: clays; dye aggregates; mixed aggregates.
INTRODUCTION
Clay minerals are some of the most profuse components of soil, leading to extensive studies from technological and environmental points of view. Studies of the properties of clays are essential for understanding the functions of clays in many chemical processes (catalysts, adsorbents, hosts for intercalation of organic molecules, etc.) (1). Many different types of clays occur in nature, like montmorillonite, hectorite, and kaolin. The first two are specified as 2:1 layered clays (with tetrahedric:octahedric:tetrahedric layers) and swell in water, whereas kaolin is a 1:1 clay (with tetrahedric:octahedric layers) and does not swell (2). The 1
To whom correspondence should be addressed.
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swelling nature of montmorillonite and hectorite makes them appropriate for colloidal studies. Laponite, a synthetic clay of lithium magnesium silicate, which forms excellent colloids in water, is also frequently used in this type of study. The particles of this clay are significantly smaller than those of SWy-1 when in aqueous suspensions. Also, the size of the lamellae, as well as the number of sheets that form the particles, are smaller than for SWy-1. It is reported that in aqueous suspensions, Laponite particles are completely dispersed, not forming tactoids, so that the clay is found mainly as individual sheets (3). The aging of these suspensions leads to the aggregation of the clay particles. An important property of the clays is that the layers are negatively charged due to the isomorphic replacement of some cations present in the clay structure by others of lower charge and similar size. This negative charge is normally balanced by hydrated cations placed in the interlayer spaces (4, 5). Cationic dyes, like those used in this work, can be attracted toward the anionic layers and are, therefore, quite suitable for investigating the properties of these minerals in aqueous suspensions (6–8). The fact that cationic dyes show metachromatic behavior even at low concentrations when in aqueous clay suspensions led Bergman and O’Konski (9) and many other authors to use dyes like methylene blue to investigate the surface/ interlayer properties of the clays. Most of these studies were done using spectrophotometric techniques (10, 11, 12). An important characteristic of these systems is that the absorption spectra of the dye changes significantly as time passes. Mechanisms based on the interpretation of these spectral variations were proposed in order to understand the processes that occur when dyes are added to the clay suspensions (6, 13, 14). Few studies of this type on systems with two different cationic dyes have been reported. Breen and Rock (15) used methylene blue as a probe to examine how the interactions between the dye and the surfaces of Na / - and H / -montmorillonites were affected by the presence of a second dye like
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thioflavin T (TFT), proflavine (PFH), or acridine yellow (ACY). These authors proposed that the initial adsorption of MB on the clay surface occurs as trimers (MB / )3 and that this species is redistributed via collisions between the clay particles until equilibrium between the different dye species is reached. Depending on the dye concentration, the protonated species MBH 2/ (at low loading) or trimers (at higher loading) will predominate in the equilibrium. The presence of a second dye, which competes for the H / sites of the clay, slowed down the approach to the equilibrium and reduced significantly the amount of MBH 2/ , proving the presence of monomeric MB / , dimeric (MB / )2 , and trimeric (MB / )3 . Moreover, the presence of PFH and ACY, which are structurally similar to MB, resulted in a larger amount of dimers. Although these authors observed also a red shift in the lmax of MB in the presence of a second dye, the possible formation of mixed aggregates was not considered. Margulies et al. (16) studied the competitive adsorption of MB, TFT, and caesium on a montmorillonite clay. They observed that TFT adsorbs strongly on the clay and, depending on the preparation procedure of the sample, TFT molecules would displace MB from the clay surface. A model for the adsorption of these organic cations onto negatively charged clays was proposed. In a later work, Margulies et al. (17) studied the competitive adsorption of MB and crystal violet (CV) on a Wyoming montmorillonite suspension and observed that MB is adsorbed to a larger extent than CV, in contrast to that expected from the individual adsorption isotherms of each dye. Pal and Schubert (18) studied the interactions between different dyes and a polyelectrolyte ( a-carragean) in aqueous solution and observed that the spectra of the solution containing both dyes were different from the sum of the spectra of the solutions of the individual dyes. These differences were ascribed to the formation of mixed aggregates induced by the polyelectrolyte chain. Oliveira et al. (19) studied the formation of mixed aggregates in homogeneous aqueous solution. A mathematical approach was developed to determine the absorption spectra of mixed dimers. The authors found that the formation constants of the mixed aggregates were systematically higher than those corresponding to the dimerization of the individual dyes. Continuing the research initiated in our laboratory on the temporal behavior of dyes in clay suspensions (6, 13, 20), we present here results obtained for systems including two dyes in the presence of clays. The detection of mixed aggregates (whose absorption spectra are well known) provides information about the processes occurring in these systems. EXPERIMENTAL
The cationic dyes methylene blue (MB, Carlo Erba p.a.) and acridine orange (AO, Aldrich) were used as received.
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The clays were SWy-1, a natural montmorillonite, obtained from the Source Clays, Clay Minerals Society, University of Missouri, and Laponite RD, a synthetic hectorite from Laporte Industries. These clays were purified as described in an earlier work (6). Some physical properties of the clays are shown in Table 1. Stock solutions of the dyes (3 1 10 05 M) were prepared using Millipore water. The clay stock suspensions (1.65 g/ L) were also prepared in Millipore deionized water and stirred until colloidal suspensions were obtained (approximately 4 h). The actual clay and dye concentrations used in each experiment are shown in the figure captions. UV-vis spectra were determined on a Hitachi U-2000 spectrophotometer interfaced to a PC computer, using 1 1 1-cm polyacrylate cuvettes (Sigma). All measurements were performed at room temperature, 25 { 17C. In order to assure the same conditions when comparing the features of the different spectra were prepared, a standardized procedure was developed to prepare the samples and make the spectrophotometric measurements of the claydye suspensions. Experiments were performed in two different ways: (a) adding an aqueous solution of both dyes to the clay suspension (Method A); and (b) mixing two suspensions, each one with one of the dyes already added to the clay (Method B). The suspensions using Method A were prepared by mixing quickly a solution of both dyes (1.0 ml of the stock solution of each dye in 3.0 ml of water) with the clay suspension (4.0 ml of the clay stock suspension in 3.0 ml of water). For Method B, two previously prepared suspensions formed by mixing 1.0 ml of the stock solution of one of the dyes, 2.0 ml of the clay stock suspension, and 3.0 ml of water were put together rapidly. The ‘‘individual suspensions’’ of each dye, used for comparison, were prepared adding 1.0 ml of the dye stock solution to 2.0 ml of clay stock suspension in 3.0 ml of water. To avoid interference due to light scattering by the clay particles, a reference sample was prepared with 2.0 ml of clay suspension in 4.0 ml of water. Differential spectra were recorded using two cuvettes with the system in the analyzing beam and two cuvettes with the individual dye-clay suspensions in the reference beam. RESULTS AND DISCUSSION
Methylene blue and acridine orange on SWy-1. The behavior of MB alone in SWy-1 suspensions was described in a previous publication (6). When MB (at the concentration used in this work) is added to SWy-1 aqueous suspensions, the initial spectrum presents a very intense peak at 580 nm corresponding to trimers and higher aggregates of the dye. Also, a small peak can be observed in the region around 670 nm, ascribed to the monomer. Significant changes in the spectrum are observed after a few minutes. The bands
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TABLE 1 Properties of the Clays Clay
Type
SiO2 (%)
Al2O3 (%)
MgO (%)
Fe2O3 (%)
Area (m2/g)
CEC (meq/100 g)
SWy-1a Laponite RDb
Natural montmorillonite Synthetic hectorite
66.9 55.6
19.6 0.08
3.05 25.1
3.35 0.04
32 360
76.4 73.3
a b
From Ref. 3. From Ref. 24.
around 670 nm (monomer) and 760 nm (diprotonated monomer) become more intense, simultaneous to a decrease in the intensity of the band around 570 nm, indicating that reorganization of the dye molecules takes place on the clay particles. As observed for MB, AO also forms aggregates on the clay surfaces, although the changes in the spectra with time were not as significant as those observed in the MB/SWy1 systems. The spectral changes of the AO-clay mixtures are more difficult to analyze as several species that absorb around the same wavelength may be present in these systems (21). The absorption wavelengths for monomers and dimers in aqueous solution (at 492 and 470 nm) are well documented (22). Experiments with AO in the presence of clays (10, 21) show absorptions at 500, 495, and 490 nm, which are assigned to dye monomers on the external surface of the clay particles, internal monomers, and internal diprotonated monomers, respectively. A similar situation was observed for the same dye in the presence of polyelectrolytes (23). In any case, the processes that occur with AO molecules should be similar to those observed for MB. The aggregated dye molecules, formed initially when the dye is added to the clay suspension, will migrate to the interlamelar region as monomers, where protonation occurs. Simultaneous adsorption of methylene blue and acridine orange on SWy-1 suspensions. Figure 1 shows the initial spectrum (at zero time) of the suspension with the mixture of the dyes (obtained according to Method A) and the sum of the spectra of the suspensions of each dye individually. It is clearly seen from these spectra, as well as from the differential spectrum shown in Fig. 2, that there is a large deviation from additivity. These deviations appear mainly in the region where the higher aggregates of MB absorb (580 nm). The sum of the individual absorption intensities around 570–580 nm is larger than those in the mixture spectrum. This is confirmed by the negative D A observed in the differential spectra in this region. The initial process, when a dye is added to the clay suspension, is the adsorption of the dye molecules on the external surface of the particles. This increases significantly the local concentration, giving rise to the formation of aggregates of the dye. When a solution containing both dyes is added to the clay suspension, the dyes are adsorbed on the clay particles,
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competing for the adsorption sites and keeping their tendency to form aggregates. In this case, the different dyes can interact with each other to form mixed aggregates, which have different absorption spectra. These results are very similar to those obtained by Pal and Schubert for the aggregation of the same dyes (MB and AO) in an anionic polyelectrolyte solution (18). These authors attributed the nonadditivity of the spectra to the formation of mixed aggregates induced by the polyelectrolyte. The formation of mixed dimers of MB-AO was also observed by Oliveira et al. (19) in homogeneous aqueous solution. Again, this was evidenced by the difference between the spectra of the mixture and the sum of those of the individual dyes. The maximum absorption wavelength of the mixed dimer was found in the 680 nm region. Similar qualitative experiments were performed by Pal and Schubert (18) with the same dyes, but using concentrations 10 times higher. The differential spectra obtained in both studies are different: in the former investigation, the ‘‘valley’’ is in the 600–680 nm region; in the latter it is displaced to shorter wavelengths and appears in the 575–580 nm region. This difference is
FIG. 1. Absorption spectra of the mixture ( —) of MB-AO [5.0 1 10 06 M] in a SWy-1 suspension [0.55 g/L] prepared by Method A and sum ( s ) of the individual suspensions of each dye, at zero time.
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FIG. 2. Differential absorption spectra of MB-AO on SWy-1 prepared by Method A at zero ( —), 180 min ( s ), and 24 h ( l ).
due to the much higher concentrations used by Pal and Schubert. In that study the formation of mixed aggregates involved the consumption of dimers and higher aggregates, whereas in the work by Oliveira et al., only monomers and dimers were consumed. To compare these results with those obtained in this work it is necessary to keep in mind that the local concentrations of the dyes are significantly higher in heterogeneous systems, increasing the possibility of formation of mixed aggregates with the general formula [(MB)n (AO)m ]. Interesting conclusions can be drawn from the temporal behavior of the differential spectrum, as shown in Fig. 2. Immediately after the solutions are mixed, a large decrease in D A is observed in the 580 nm region, as well as positive peaks around the 480–490 and 680–700 nm regions. After 180 min, the valley at 580 nm practically disappears and the D A around 480– 490 and 670 nm also decreases. At the same time, new absorptions appear in the 500, 700, and 750 nm regions. The initial behavior around 580 nm, as pointed out above, reflects a reduction in the amount of higher self-aggregates of MB present in the suspensions with the mixture of dyes, because of the formation of mixed aggregates. These mixed aggregates will absorb at the wavelength of the positive peaks observed at 480 and 690 nm. When left for a time, both the mixed aggregates and the higher (self-) aggregates of MB, formed in the mixture and in the individual suspension respectively, will deaggregate and redistribute on the clay surface, eventually migrating to the interlamelar spaces where they will be present as protonated monomers. Nevertheless, if just reorganization processes of the dye molecules on the clay surface should take place, only the decrease of the difference between the absorption spectra of the mixture and the sum should be observed as time passes. However, the growth of new peaks at 500, 700, and 750 nm indicates that different processes are occurring in the mixture, compared to the individual suspension of the dyes. The
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peaks at 500 and 700 nm, placed at wavelengths larger and shorter than those for the adsorbed monomers of MB and AO, may correspond to the mixed aggregates formed in ‘‘internal regions’’ as a result of the interactions of the clay particles. This is possible as at longer times an aging process will certainly occur, allowing the clay particles to approximate and associate to each other, leading to flocculation. The growth of the peak at 750 nm is probably due to a larger quantity of protonated monomers of MB formed in the mixture suspension, compared to that formed in the individual suspension of MB. The initial spectrum of the sample prepared according to Procedure B, that is, mixing suspensions of each dye already adsorbed on the clay, is practically identical to the sum of the spectra of the suspensions of the separate dyes. It would be practically impossible to observe any difference between those spectra immediately after mixing as there is not enough time for any absorption-desorption processes of the dye or aggregation of the clay particles to occur. Analyses of these spectra at different times show the occurrence of changes, which can be ascribed to interactions between clay particles. Comparison of the differential spectra of Experiments A and B (Figs. 2 and 3, respectively) after 24 h shows that both spectra are similar, even for their absorption intensities. This indicates that the temporal evolution of each system and the interaction of the clay particles containing dyes lead to equivalent configurations after sufficient time. Scheme 1 illustrates the processes that occur in the dyeclay suspensions prepared by Method A. It can be observed that when both dyes are added to a SWy-1 suspension simultaneously, the first step is the formation of external selfaggregates, as well as mixed aggregates MB-AO, that absorb around 480 and 695–700 nm. As time passes, a small shift of the absorption maximum at 480 nm to higher wavelengths can be detected. Initially the absorption is due to external
FIG. 3. Differential absorption spectra of MB-AO on SWy-1 prepared by Method B at zero ( —), 180 min ( s ), and 24 h ( l ).
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FIG. 4. Differential spectrum of MB-AO on Laponite at zero ( —), 180 min ( s ), and 24 h ( l ). Suspensions prepared according to Method A.
mixed aggregates that absorb at those wavelengths, but at longer times due to the aging of the clay, the aggregates will turn into internal mixed aggregates absorbing at higher wavelengths (around 500 nm). Later, due to the rearrangements of the dye molecules on the clay particles, the aggregates (self- and mixed aggregates) tend to disappear, and the absorption intensity around 480 nm is reduced. Now, monomers can migrate to the interlamelar space, where they can protonate. At longer times, when the association of the clay particles becomes an important process, bands appear around 500 and 690–700 nm. These bands correspond to the mixed aggregates localized on the internal surface originated by the particle-particle association. The aggregation constants for the mixed species (mixed dimer) are known to be higher than those for self-aggregation of the separate dyes (19). This is due to the larger charge-transfer contribution to the attractive interaction when both units have different electron densities. Therefore, assuming that the same tendency is maintained when the dyes are adsorbed on the clay particles, the amount of mixed aggregates should be higher than that of self-aggregates. The processes occurring in mixtures prepared according to Method B are similar, except that no mixed aggregates are formed immediately after the solutions are mixed. Nevertheless, these aggregates will be formed at longer times, proving that the same configuration is obtained after enough time, independently of the experimental procedure used to form the clay-dye systems. There is also a possibility for the external self-aggregates of AO and MB to be trapped in the internal spaces formed by the association of clay particles. However, this process will occur only in small proportion because the amount of external aggregates is reduced at longer times. Methylene blue - acridine orange on Laponite suspensions. Similar experiments (A and B) were performed with
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the same pair of dyes (MB and AO) on Laponite suspensions. The temporal behavior of MB adsorbed on Laponite is significantly different from SWy-1 suspensions, as can be observed from the evolution of the absorption spectra with time (6). In the case of MB on Laponite, the initial step is predominantly the adsorption of monomers. The formation of aggregates is considerably lower because of the much smaller size of the Laponite particles, which results in a larger superficial area, favoring the adsorption in the monomeric form. As time passes, the growth of a new band around 655 nm is observed. It should be noted that Cenens et al. (12), studying the behavior of MB on Laponite B suspensions in buffered solutions, made an alternative assignment to the 655-nm band, attributing it to an internal monomer of MB. However, this band does not occur on the other clays, or in the absence of added electrolytes (21), so that it may be attributed to the association of the Laponite particles, forcing the dye to aggregate probably as an internal dimer placed between the clay sheets, formed due to the interactions between the dispersed particles. In contrast, only small spectral variations were observed when AO was added to Laponite suspensions. The difference between the spectrum of the mixture of the dyes in Laponite suspensions prepared by Method A with that of the separate dyes is not as significant as that observed for the systems with SWy-1, at similar dye and clay concentrations. The differential spectra are also different from those of Fig. 2. The absorbance of systems containing Laponite is mainly due to dyes adsorbed as monomers, as a consequence of the larger superficial area available for adsorption in this clay. Thus, when the dyes are added individually to the suspension, the absorption due to monomers prevails, but when both dyes are added simultaneously some mixed aggregates will be formed at the expense of dye monomers, resulting in the differences observed in the differential spectra. This explains the valley around 670 nm observed at zero time in Fig. 4. As noted before, the tendency for the dye molecules to form mixed aggregates must be higher, because of the larger aggregation constants compared to those for the self-aggregates, as found in homogeneous media (19). As time passes, the depth of the valley increases and is displaced toward lower wavelengths, whereas an increase in the positive D A values is observed around 500 and 695 nm. As a result of the association of the clay particles in the individual suspensions, self-dimers are formed in the internal regions created by the association of the clays. In the case of MB these internal dimers absorb around 655 nm (6) while the mixed dimers formed in the mixture have higher absorption intensities at the regions of positive D A values. These associating processes are also confirmed by experiments performed according to Method B, in which small differences between the individual and the mixture spectra
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FIG. 5. Differential absorption spectra of MB-AO on Laponite at zero ( —) 180 min ( s ), and 24 h ( l ). Suspensions prepared according to Method B.
can be detected even at zero time measurements. These differences appear clearly in the differential spectra in Fig. 5. There are positive D A’s around 500 and 670 nm, originated by the mixed species formed due to the particle-particle interactions that take place even at very short times, immediately after the mixing of the suspensions containing the dyes. The time interval between the sample preparation and the actual measurement seems to be enough to detect the observed changes. When the clay particles are covered by dye molecules the tendency of the particles to aggregate increases, because the unbalanced superficial charges are neutralized by the positive charges of the dye molecules, decreasing the electrostatic repulsion between the clay particles. This favors the associating process, as can be observed for the suspensions prepared according to Method B. When the suspensions of each dye are mixed, the association of the particles containing differ-
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ent dyes still happens, and the available superficial area decreases, forcing the dyes to aggregate. Therefore, mixed aggregates will also be formed, as evidenced in the differential spectra. Practically, no valleys were observed at zero time in experiment with mixtures prepared according to Method B (Fig. 5). As the dyes are already adsorbed on the clay prior to the mixture, the same processes will be occurring in suspensions with the mixture and with the individual dyes, i.e., the formation of self-aggregates of MB and AO. No significant changes are observed in the differential spectra in the regions corresponding to these aggregates. Larger differences were observed only at longer times. It should be noted also that after 24 h the differential absorption spectra are very similar to those obtained in experiments with Method A. This suggests that the temporal evolution in each system due to the interactions between the clay particles containing the dyes leads to similar final situations, as was already observed for SWy-1. Scheme 2 illustrates the processes described above. It should be noted that when both dyes are added simultaneously to a Laponite suspension (Experiment A), mixed aggregates are immediately formed on the external surface of the clay particle. The amount of these aggregates is lower than in systems containing SWy-1. This scheme shows also that in Laponite suspensions, different than those in systems with SWy-1, the spectral changes with time are mainly due to the processes of association of Laponite particles. Also, due to the larger available area of Laponite, the dyes are initially adsorbed as monomers. CONCLUSIONS
Dye molecules added to clay suspensions are adsorbed and aggregate on the clay particles. The absorption spectra of suspensions containing two different dyes are different from the sum of those of the suspensions of the individual
SCHEME 1. Processes occurring on addition of dyes A and B to SWy-1. A corresponds to AO and B to MB. Broken arrows represent processes observed only when the samples were prepared according to Method A.
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SCHEME 2. Processes occurring on addition of dyes A and B to Laponite. A corresponds to AO and B to MB.
dyes. The nonadditivity of the spectra, also observed in the differential spectra, is ascribed to the formation of mixed aggregates. In systems containing SWy-1, the spectral variations as function of time are explained by the predominance of reorganization processes of the dye molecules that start to occur immediately after the dye solutions are mixed with the clay suspension. On the other hand, for systems containing Laponite, the variations are mainly due to the association-dissociation processes of the clay particles with time. At longer times, clay association processes are also responsible for significant spectral changes in systems containing SWy-1. ACKNOWLEDGMENTS Financial support by FAPESP ( 94 / 3505 ) and FINEP ( 65 / 92.0063.00 ) is gratefully acknowledged. A.P.P.C. thanks FAPESP for a graduate fellowship.
REFERENCES 1. (a) Khan, S. T., ‘‘Pesticides in the Soil Environment,’’ Fundamental Aspects of Pollution Control and Environmental Science 5. Elsevier, New York, 1980; (b) Barrer, R. M., ‘‘Zeolites and Clay Minerals as Sorbents and Molecular Sieves.’’ Academic Press, London, 1978; (c) Thomas, J. M., ‘‘Intercalation Chemistry,’’ p 55. Academic Press, London, 1982. 2. (a) Thomas, J. K., Chem. Rev. 93, 301 (1993); (b) Theng, B. K. G., ‘‘The Chemistry of Clay Organic Reactions,’’ Chap. 1. Wiley, New York, 1974.
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3. Van Olphen, H., and Fripiat, J. J. (Eds.), ‘‘Data Handbook for Clay Materials and Other Non-Metallic Minerals.’’ Pergamon Press, Oxford, 1979. 4. Thomas, J. K., J. Phys. Chem. 91, 267 (1987). 5. Thomas, J. K., ‘‘The Chemistry of Excitation at Interfaces,’’ ACS Monograph 191. ACS, Washington, DC, 1984. 6. Gessner, F., Schmitt, C. C., and Neumann, M. G., Langmuir 10, 3749 (1994). 7. Cenens, J., and Schoonheydt, R. A., Clays Clay Miner. 36, 214 (1988). 8. (a) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 553 (1971); (b) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 537 (1971). 9. Bergman, K., and O’Konski, C. T., J. Phys. Chem. 67, 2169 (1963). 10. Cohen, R., and Yariv, S., J. Chem. Soc. Faraday Trans. 1 80, 1705 (1984). 11. Thomas, J. K., Acc. Chem. Res. 21, 275 (1988). 12. (a) Cenens, J., Schoonheydt, R. A., and De Schryver, F. C., ACS Symp. Ser. 415, 378 (1989); (b) Cenens, J., and Schoonheydt, R. A., Clays Clay Miner. 36, 214 (1988). 13. Neumann, M. G., Schmitt, C. C., and Gessner, F., J. Colloid Interface Sci. 177, 495 (1996). 14. Tapia Estevez, M. J., Lo´pez Arbeloa, F., Lo´pez Arbeloa, T., and Lo´pez Arbeloa, I., J. Colloid Interface Sci. 162, 412 (1994). 15. Breen, C., and Rock, B., Clay Miner. 29, 179 (1994). 16. Margulies, L., Rozen, H., and Nir, S., Clays Clay Miner. 36, 270 (1980). 17. Margulies, L., Nir, S., and Rytwo, G., Clay Miner. 28, 139 (1993). 18. Pal, M. K., and Schubert, M., J. Phys. Chem. 67, 1821 (1963). 19. Oliveira, V. A., Neumann, M. G., and Gessner, F., J. Chem. Soc. Faraday Trans. 86, 3551 (1990). 20. Gessner, F., Cione, A. P., and Neumann, M. G. [in press] 21. Gessner, F. [in press] 22. Lamm, M. E., and Neville, D. M., Jr., J. Phys. Chem. 69, 3872 (1965). 23. Vitagliano, V. in ‘‘Aggregation Processes in Solution’’ (E. Wyn-Jones, and J. Gormally, Eds.), Chap. 11, p. 271. Elsevier, New York, 1983. 24. Mao, P., and Thomas, J. K., Langmuir 9, 1504 (1993).
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CS975268
Time-Dependent Spectrophotometric Study of the Interaction of Basic Dyes with Clays III. Mixed Dye Aggregates on SWy-1 and Laponite Ana P. P. Cione, Miguel G. Neumann, 1 and Fergus Gessner 1 Instituto de QuıB mica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Caixa Postal 780, 13560-970 Sa˜o Carlos SP, Brazil Received July 7, 1997; accepted October 24, 1997
The interactions of a pair of different cationic dyes (methylene blue and acridine orange) in the presence of dilute aqueous suspensions of the montmorillonite SWy-1 and Laponite clays were studied using spectrophotometric techniques. A standardized procedure was developed to prepare the samples and perform the spectrophotometric measurements of the clay-dye suspensions. The observed nonadditivity of the absorption spectra of the mixture compared with those of the individual dyes is attributed to the formation of mixed aggregates on the clay particles. Significant spectral variations with time were observed: for suspensions containing the montmorillonite SWy-1 they were mainly due to the reorganization of the dye molecules on the clay particles. For systems containing Laponite, the spectral changes were ascribed to processes of approximation-association of the clay particles. At longer times (24 h) the association processes prevail, even for the SWy-1 suspensions. After sufficient time, both the SWy-1 and the Laponite suspensions showed similar spectra, indicating that the processes occurring in both systems lead to similar configurations. Schemes are proposed to describe the temporal variations occurring in these systems. q 1998 Academic Press Key Words: clays; dye aggregates; mixed aggregates.
INTRODUCTION
Clay minerals are some of the most profuse components of soil, leading to extensive studies from technological and environmental points of view. Studies of the properties of clays are essential for understanding the functions of clays in many chemical processes (catalysts, adsorbents, hosts for intercalation of organic molecules, etc.) (1). Many different types of clays occur in nature, like montmorillonite, hectorite, and kaolin. The first two are specified as 2:1 layered clays (with tetrahedric:octahedric:tetrahedric layers) and swell in water, whereas kaolin is a 1:1 clay (with tetrahedric:octahedric layers) and does not swell (2). The 1
To whom correspondence should be addressed.
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0021-9797/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
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swelling nature of montmorillonite and hectorite makes them appropriate for colloidal studies. Laponite, a synthetic clay of lithium magnesium silicate, which forms excellent colloids in water, is also frequently used in this type of study. The particles of this clay are significantly smaller than those of SWy-1 when in aqueous suspensions. Also, the size of the lamellae, as well as the number of sheets that form the particles, are smaller than for SWy-1. It is reported that in aqueous suspensions, Laponite particles are completely dispersed, not forming tactoids, so that the clay is found mainly as individual sheets (3). The aging of these suspensions leads to the aggregation of the clay particles. An important property of the clays is that the layers are negatively charged due to the isomorphic replacement of some cations present in the clay structure by others of lower charge and similar size. This negative charge is normally balanced by hydrated cations placed in the interlayer spaces (4, 5). Cationic dyes, like those used in this work, can be attracted toward the anionic layers and are, therefore, quite suitable for investigating the properties of these minerals in aqueous suspensions (6–8). The fact that cationic dyes show metachromatic behavior even at low concentrations when in aqueous clay suspensions led Bergman and O’Konski (9) and many other authors to use dyes like methylene blue to investigate the surface/ interlayer properties of the clays. Most of these studies were done using spectrophotometric techniques (10, 11, 12). An important characteristic of these systems is that the absorption spectra of the dye changes significantly as time passes. Mechanisms based on the interpretation of these spectral variations were proposed in order to understand the processes that occur when dyes are added to the clay suspensions (6, 13, 14). Few studies of this type on systems with two different cationic dyes have been reported. Breen and Rock (15) used methylene blue as a probe to examine how the interactions between the dye and the surfaces of Na / - and H / -montmorillonites were affected by the presence of a second dye like
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thioflavin T (TFT), proflavine (PFH), or acridine yellow (ACY). These authors proposed that the initial adsorption of MB on the clay surface occurs as trimers (MB / )3 and that this species is redistributed via collisions between the clay particles until equilibrium between the different dye species is reached. Depending on the dye concentration, the protonated species MBH 2/ (at low loading) or trimers (at higher loading) will predominate in the equilibrium. The presence of a second dye, which competes for the H / sites of the clay, slowed down the approach to the equilibrium and reduced significantly the amount of MBH 2/ , proving the presence of monomeric MB / , dimeric (MB / )2 , and trimeric (MB / )3 . Moreover, the presence of PFH and ACY, which are structurally similar to MB, resulted in a larger amount of dimers. Although these authors observed also a red shift in the lmax of MB in the presence of a second dye, the possible formation of mixed aggregates was not considered. Margulies et al. (16) studied the competitive adsorption of MB, TFT, and caesium on a montmorillonite clay. They observed that TFT adsorbs strongly on the clay and, depending on the preparation procedure of the sample, TFT molecules would displace MB from the clay surface. A model for the adsorption of these organic cations onto negatively charged clays was proposed. In a later work, Margulies et al. (17) studied the competitive adsorption of MB and crystal violet (CV) on a Wyoming montmorillonite suspension and observed that MB is adsorbed to a larger extent than CV, in contrast to that expected from the individual adsorption isotherms of each dye. Pal and Schubert (18) studied the interactions between different dyes and a polyelectrolyte ( a-carragean) in aqueous solution and observed that the spectra of the solution containing both dyes were different from the sum of the spectra of the solutions of the individual dyes. These differences were ascribed to the formation of mixed aggregates induced by the polyelectrolyte chain. Oliveira et al. (19) studied the formation of mixed aggregates in homogeneous aqueous solution. A mathematical approach was developed to determine the absorption spectra of mixed dimers. The authors found that the formation constants of the mixed aggregates were systematically higher than those corresponding to the dimerization of the individual dyes. Continuing the research initiated in our laboratory on the temporal behavior of dyes in clay suspensions (6, 13, 20), we present here results obtained for systems including two dyes in the presence of clays. The detection of mixed aggregates (whose absorption spectra are well known) provides information about the processes occurring in these systems. EXPERIMENTAL
The cationic dyes methylene blue (MB, Carlo Erba p.a.) and acridine orange (AO, Aldrich) were used as received.
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The clays were SWy-1, a natural montmorillonite, obtained from the Source Clays, Clay Minerals Society, University of Missouri, and Laponite RD, a synthetic hectorite from Laporte Industries. These clays were purified as described in an earlier work (6). Some physical properties of the clays are shown in Table 1. Stock solutions of the dyes (3 1 10 05 M) were prepared using Millipore water. The clay stock suspensions (1.65 g/ L) were also prepared in Millipore deionized water and stirred until colloidal suspensions were obtained (approximately 4 h). The actual clay and dye concentrations used in each experiment are shown in the figure captions. UV-vis spectra were determined on a Hitachi U-2000 spectrophotometer interfaced to a PC computer, using 1 1 1-cm polyacrylate cuvettes (Sigma). All measurements were performed at room temperature, 25 { 17C. In order to assure the same conditions when comparing the features of the different spectra were prepared, a standardized procedure was developed to prepare the samples and make the spectrophotometric measurements of the claydye suspensions. Experiments were performed in two different ways: (a) adding an aqueous solution of both dyes to the clay suspension (Method A); and (b) mixing two suspensions, each one with one of the dyes already added to the clay (Method B). The suspensions using Method A were prepared by mixing quickly a solution of both dyes (1.0 ml of the stock solution of each dye in 3.0 ml of water) with the clay suspension (4.0 ml of the clay stock suspension in 3.0 ml of water). For Method B, two previously prepared suspensions formed by mixing 1.0 ml of the stock solution of one of the dyes, 2.0 ml of the clay stock suspension, and 3.0 ml of water were put together rapidly. The ‘‘individual suspensions’’ of each dye, used for comparison, were prepared adding 1.0 ml of the dye stock solution to 2.0 ml of clay stock suspension in 3.0 ml of water. To avoid interference due to light scattering by the clay particles, a reference sample was prepared with 2.0 ml of clay suspension in 4.0 ml of water. Differential spectra were recorded using two cuvettes with the system in the analyzing beam and two cuvettes with the individual dye-clay suspensions in the reference beam. RESULTS AND DISCUSSION
Methylene blue and acridine orange on SWy-1. The behavior of MB alone in SWy-1 suspensions was described in a previous publication (6). When MB (at the concentration used in this work) is added to SWy-1 aqueous suspensions, the initial spectrum presents a very intense peak at 580 nm corresponding to trimers and higher aggregates of the dye. Also, a small peak can be observed in the region around 670 nm, ascribed to the monomer. Significant changes in the spectrum are observed after a few minutes. The bands
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TABLE 1 Properties of the Clays Clay
Type
SiO2 (%)
Al2O3 (%)
MgO (%)
Fe2O3 (%)
Area (m2/g)
CEC (meq/100 g)
SWy-1a Laponite RDb
Natural montmorillonite Synthetic hectorite
66.9 55.6
19.6 0.08
3.05 25.1
3.35 0.04
32 360
76.4 73.3
a b
From Ref. 3. From Ref. 24.
around 670 nm (monomer) and 760 nm (diprotonated monomer) become more intense, simultaneous to a decrease in the intensity of the band around 570 nm, indicating that reorganization of the dye molecules takes place on the clay particles. As observed for MB, AO also forms aggregates on the clay surfaces, although the changes in the spectra with time were not as significant as those observed in the MB/SWy1 systems. The spectral changes of the AO-clay mixtures are more difficult to analyze as several species that absorb around the same wavelength may be present in these systems (21). The absorption wavelengths for monomers and dimers in aqueous solution (at 492 and 470 nm) are well documented (22). Experiments with AO in the presence of clays (10, 21) show absorptions at 500, 495, and 490 nm, which are assigned to dye monomers on the external surface of the clay particles, internal monomers, and internal diprotonated monomers, respectively. A similar situation was observed for the same dye in the presence of polyelectrolytes (23). In any case, the processes that occur with AO molecules should be similar to those observed for MB. The aggregated dye molecules, formed initially when the dye is added to the clay suspension, will migrate to the interlamelar region as monomers, where protonation occurs. Simultaneous adsorption of methylene blue and acridine orange on SWy-1 suspensions. Figure 1 shows the initial spectrum (at zero time) of the suspension with the mixture of the dyes (obtained according to Method A) and the sum of the spectra of the suspensions of each dye individually. It is clearly seen from these spectra, as well as from the differential spectrum shown in Fig. 2, that there is a large deviation from additivity. These deviations appear mainly in the region where the higher aggregates of MB absorb (580 nm). The sum of the individual absorption intensities around 570–580 nm is larger than those in the mixture spectrum. This is confirmed by the negative D A observed in the differential spectra in this region. The initial process, when a dye is added to the clay suspension, is the adsorption of the dye molecules on the external surface of the particles. This increases significantly the local concentration, giving rise to the formation of aggregates of the dye. When a solution containing both dyes is added to the clay suspension, the dyes are adsorbed on the clay particles,
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competing for the adsorption sites and keeping their tendency to form aggregates. In this case, the different dyes can interact with each other to form mixed aggregates, which have different absorption spectra. These results are very similar to those obtained by Pal and Schubert for the aggregation of the same dyes (MB and AO) in an anionic polyelectrolyte solution (18). These authors attributed the nonadditivity of the spectra to the formation of mixed aggregates induced by the polyelectrolyte. The formation of mixed dimers of MB-AO was also observed by Oliveira et al. (19) in homogeneous aqueous solution. Again, this was evidenced by the difference between the spectra of the mixture and the sum of those of the individual dyes. The maximum absorption wavelength of the mixed dimer was found in the 680 nm region. Similar qualitative experiments were performed by Pal and Schubert (18) with the same dyes, but using concentrations 10 times higher. The differential spectra obtained in both studies are different: in the former investigation, the ‘‘valley’’ is in the 600–680 nm region; in the latter it is displaced to shorter wavelengths and appears in the 575–580 nm region. This difference is
FIG. 1. Absorption spectra of the mixture ( —) of MB-AO [5.0 1 10 06 M] in a SWy-1 suspension [0.55 g/L] prepared by Method A and sum ( s ) of the individual suspensions of each dye, at zero time.
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FIG. 2. Differential absorption spectra of MB-AO on SWy-1 prepared by Method A at zero ( —), 180 min ( s ), and 24 h ( l ).
due to the much higher concentrations used by Pal and Schubert. In that study the formation of mixed aggregates involved the consumption of dimers and higher aggregates, whereas in the work by Oliveira et al., only monomers and dimers were consumed. To compare these results with those obtained in this work it is necessary to keep in mind that the local concentrations of the dyes are significantly higher in heterogeneous systems, increasing the possibility of formation of mixed aggregates with the general formula [(MB)n (AO)m ]. Interesting conclusions can be drawn from the temporal behavior of the differential spectrum, as shown in Fig. 2. Immediately after the solutions are mixed, a large decrease in D A is observed in the 580 nm region, as well as positive peaks around the 480–490 and 680–700 nm regions. After 180 min, the valley at 580 nm practically disappears and the D A around 480– 490 and 670 nm also decreases. At the same time, new absorptions appear in the 500, 700, and 750 nm regions. The initial behavior around 580 nm, as pointed out above, reflects a reduction in the amount of higher self-aggregates of MB present in the suspensions with the mixture of dyes, because of the formation of mixed aggregates. These mixed aggregates will absorb at the wavelength of the positive peaks observed at 480 and 690 nm. When left for a time, both the mixed aggregates and the higher (self-) aggregates of MB, formed in the mixture and in the individual suspension respectively, will deaggregate and redistribute on the clay surface, eventually migrating to the interlamelar spaces where they will be present as protonated monomers. Nevertheless, if just reorganization processes of the dye molecules on the clay surface should take place, only the decrease of the difference between the absorption spectra of the mixture and the sum should be observed as time passes. However, the growth of new peaks at 500, 700, and 750 nm indicates that different processes are occurring in the mixture, compared to the individual suspension of the dyes. The
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peaks at 500 and 700 nm, placed at wavelengths larger and shorter than those for the adsorbed monomers of MB and AO, may correspond to the mixed aggregates formed in ‘‘internal regions’’ as a result of the interactions of the clay particles. This is possible as at longer times an aging process will certainly occur, allowing the clay particles to approximate and associate to each other, leading to flocculation. The growth of the peak at 750 nm is probably due to a larger quantity of protonated monomers of MB formed in the mixture suspension, compared to that formed in the individual suspension of MB. The initial spectrum of the sample prepared according to Procedure B, that is, mixing suspensions of each dye already adsorbed on the clay, is practically identical to the sum of the spectra of the suspensions of the separate dyes. It would be practically impossible to observe any difference between those spectra immediately after mixing as there is not enough time for any absorption-desorption processes of the dye or aggregation of the clay particles to occur. Analyses of these spectra at different times show the occurrence of changes, which can be ascribed to interactions between clay particles. Comparison of the differential spectra of Experiments A and B (Figs. 2 and 3, respectively) after 24 h shows that both spectra are similar, even for their absorption intensities. This indicates that the temporal evolution of each system and the interaction of the clay particles containing dyes lead to equivalent configurations after sufficient time. Scheme 1 illustrates the processes that occur in the dyeclay suspensions prepared by Method A. It can be observed that when both dyes are added to a SWy-1 suspension simultaneously, the first step is the formation of external selfaggregates, as well as mixed aggregates MB-AO, that absorb around 480 and 695–700 nm. As time passes, a small shift of the absorption maximum at 480 nm to higher wavelengths can be detected. Initially the absorption is due to external
FIG. 3. Differential absorption spectra of MB-AO on SWy-1 prepared by Method B at zero ( —), 180 min ( s ), and 24 h ( l ).
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FIG. 4. Differential spectrum of MB-AO on Laponite at zero ( —), 180 min ( s ), and 24 h ( l ). Suspensions prepared according to Method A.
mixed aggregates that absorb at those wavelengths, but at longer times due to the aging of the clay, the aggregates will turn into internal mixed aggregates absorbing at higher wavelengths (around 500 nm). Later, due to the rearrangements of the dye molecules on the clay particles, the aggregates (self- and mixed aggregates) tend to disappear, and the absorption intensity around 480 nm is reduced. Now, monomers can migrate to the interlamelar space, where they can protonate. At longer times, when the association of the clay particles becomes an important process, bands appear around 500 and 690–700 nm. These bands correspond to the mixed aggregates localized on the internal surface originated by the particle-particle association. The aggregation constants for the mixed species (mixed dimer) are known to be higher than those for self-aggregation of the separate dyes (19). This is due to the larger charge-transfer contribution to the attractive interaction when both units have different electron densities. Therefore, assuming that the same tendency is maintained when the dyes are adsorbed on the clay particles, the amount of mixed aggregates should be higher than that of self-aggregates. The processes occurring in mixtures prepared according to Method B are similar, except that no mixed aggregates are formed immediately after the solutions are mixed. Nevertheless, these aggregates will be formed at longer times, proving that the same configuration is obtained after enough time, independently of the experimental procedure used to form the clay-dye systems. There is also a possibility for the external self-aggregates of AO and MB to be trapped in the internal spaces formed by the association of clay particles. However, this process will occur only in small proportion because the amount of external aggregates is reduced at longer times. Methylene blue - acridine orange on Laponite suspensions. Similar experiments (A and B) were performed with
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the same pair of dyes (MB and AO) on Laponite suspensions. The temporal behavior of MB adsorbed on Laponite is significantly different from SWy-1 suspensions, as can be observed from the evolution of the absorption spectra with time (6). In the case of MB on Laponite, the initial step is predominantly the adsorption of monomers. The formation of aggregates is considerably lower because of the much smaller size of the Laponite particles, which results in a larger superficial area, favoring the adsorption in the monomeric form. As time passes, the growth of a new band around 655 nm is observed. It should be noted that Cenens et al. (12), studying the behavior of MB on Laponite B suspensions in buffered solutions, made an alternative assignment to the 655-nm band, attributing it to an internal monomer of MB. However, this band does not occur on the other clays, or in the absence of added electrolytes (21), so that it may be attributed to the association of the Laponite particles, forcing the dye to aggregate probably as an internal dimer placed between the clay sheets, formed due to the interactions between the dispersed particles. In contrast, only small spectral variations were observed when AO was added to Laponite suspensions. The difference between the spectrum of the mixture of the dyes in Laponite suspensions prepared by Method A with that of the separate dyes is not as significant as that observed for the systems with SWy-1, at similar dye and clay concentrations. The differential spectra are also different from those of Fig. 2. The absorbance of systems containing Laponite is mainly due to dyes adsorbed as monomers, as a consequence of the larger superficial area available for adsorption in this clay. Thus, when the dyes are added individually to the suspension, the absorption due to monomers prevails, but when both dyes are added simultaneously some mixed aggregates will be formed at the expense of dye monomers, resulting in the differences observed in the differential spectra. This explains the valley around 670 nm observed at zero time in Fig. 4. As noted before, the tendency for the dye molecules to form mixed aggregates must be higher, because of the larger aggregation constants compared to those for the self-aggregates, as found in homogeneous media (19). As time passes, the depth of the valley increases and is displaced toward lower wavelengths, whereas an increase in the positive D A values is observed around 500 and 695 nm. As a result of the association of the clay particles in the individual suspensions, self-dimers are formed in the internal regions created by the association of the clays. In the case of MB these internal dimers absorb around 655 nm (6) while the mixed dimers formed in the mixture have higher absorption intensities at the regions of positive D A values. These associating processes are also confirmed by experiments performed according to Method B, in which small differences between the individual and the mixture spectra
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FIG. 5. Differential absorption spectra of MB-AO on Laponite at zero ( —) 180 min ( s ), and 24 h ( l ). Suspensions prepared according to Method B.
can be detected even at zero time measurements. These differences appear clearly in the differential spectra in Fig. 5. There are positive D A’s around 500 and 670 nm, originated by the mixed species formed due to the particle-particle interactions that take place even at very short times, immediately after the mixing of the suspensions containing the dyes. The time interval between the sample preparation and the actual measurement seems to be enough to detect the observed changes. When the clay particles are covered by dye molecules the tendency of the particles to aggregate increases, because the unbalanced superficial charges are neutralized by the positive charges of the dye molecules, decreasing the electrostatic repulsion between the clay particles. This favors the associating process, as can be observed for the suspensions prepared according to Method B. When the suspensions of each dye are mixed, the association of the particles containing differ-
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ent dyes still happens, and the available superficial area decreases, forcing the dyes to aggregate. Therefore, mixed aggregates will also be formed, as evidenced in the differential spectra. Practically, no valleys were observed at zero time in experiment with mixtures prepared according to Method B (Fig. 5). As the dyes are already adsorbed on the clay prior to the mixture, the same processes will be occurring in suspensions with the mixture and with the individual dyes, i.e., the formation of self-aggregates of MB and AO. No significant changes are observed in the differential spectra in the regions corresponding to these aggregates. Larger differences were observed only at longer times. It should be noted also that after 24 h the differential absorption spectra are very similar to those obtained in experiments with Method A. This suggests that the temporal evolution in each system due to the interactions between the clay particles containing the dyes leads to similar final situations, as was already observed for SWy-1. Scheme 2 illustrates the processes described above. It should be noted that when both dyes are added simultaneously to a Laponite suspension (Experiment A), mixed aggregates are immediately formed on the external surface of the clay particle. The amount of these aggregates is lower than in systems containing SWy-1. This scheme shows also that in Laponite suspensions, different than those in systems with SWy-1, the spectral changes with time are mainly due to the processes of association of Laponite particles. Also, due to the larger available area of Laponite, the dyes are initially adsorbed as monomers. CONCLUSIONS
Dye molecules added to clay suspensions are adsorbed and aggregate on the clay particles. The absorption spectra of suspensions containing two different dyes are different from the sum of those of the suspensions of the individual
SCHEME 1. Processes occurring on addition of dyes A and B to SWy-1. A corresponds to AO and B to MB. Broken arrows represent processes observed only when the samples were prepared according to Method A.
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SCHEME 2. Processes occurring on addition of dyes A and B to Laponite. A corresponds to AO and B to MB.
dyes. The nonadditivity of the spectra, also observed in the differential spectra, is ascribed to the formation of mixed aggregates. In systems containing SWy-1, the spectral variations as function of time are explained by the predominance of reorganization processes of the dye molecules that start to occur immediately after the dye solutions are mixed with the clay suspension. On the other hand, for systems containing Laponite, the variations are mainly due to the association-dissociation processes of the clay particles with time. At longer times, clay association processes are also responsible for significant spectral changes in systems containing SWy-1. ACKNOWLEDGMENTS Financial support by FAPESP ( 94 / 3505 ) and FINEP ( 65 / 92.0063.00 ) is gratefully acknowledged. A.P.P.C. thanks FAPESP for a graduate fellowship.
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3. Van Olphen, H., and Fripiat, J. J. (Eds.), ‘‘Data Handbook for Clay Materials and Other Non-Metallic Minerals.’’ Pergamon Press, Oxford, 1979. 4. Thomas, J. K., J. Phys. Chem. 91, 267 (1987). 5. Thomas, J. K., ‘‘The Chemistry of Excitation at Interfaces,’’ ACS Monograph 191. ACS, Washington, DC, 1984. 6. Gessner, F., Schmitt, C. C., and Neumann, M. G., Langmuir 10, 3749 (1994). 7. Cenens, J., and Schoonheydt, R. A., Clays Clay Miner. 36, 214 (1988). 8. (a) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 553 (1971); (b) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 537 (1971). 9. Bergman, K., and O’Konski, C. T., J. Phys. Chem. 67, 2169 (1963). 10. Cohen, R., and Yariv, S., J. Chem. Soc. Faraday Trans. 1 80, 1705 (1984). 11. Thomas, J. K., Acc. Chem. Res. 21, 275 (1988). 12. (a) Cenens, J., Schoonheydt, R. A., and De Schryver, F. C., ACS Symp. Ser. 415, 378 (1989); (b) Cenens, J., and Schoonheydt, R. A., Clays Clay Miner. 36, 214 (1988). 13. Neumann, M. G., Schmitt, C. C., and Gessner, F., J. Colloid Interface Sci. 177, 495 (1996). 14. Tapia Estevez, M. J., Lo´pez Arbeloa, F., Lo´pez Arbeloa, T., and Lo´pez Arbeloa, I., J. Colloid Interface Sci. 162, 412 (1994). 15. Breen, C., and Rock, B., Clay Miner. 29, 179 (1994). 16. Margulies, L., Rozen, H., and Nir, S., Clays Clay Miner. 36, 270 (1980). 17. Margulies, L., Nir, S., and Rytwo, G., Clay Miner. 28, 139 (1993). 18. Pal, M. K., and Schubert, M., J. Phys. Chem. 67, 1821 (1963). 19. Oliveira, V. A., Neumann, M. G., and Gessner, F., J. Chem. Soc. Faraday Trans. 86, 3551 (1990). 20. Gessner, F., Cione, A. P., and Neumann, M. G. [in press] 21. Gessner, F. [in press] 22. Lamm, M. E., and Neville, D. M., Jr., J. Phys. Chem. 69, 3872 (1965). 23. Vitagliano, V. in ‘‘Aggregation Processes in Solution’’ (E. Wyn-Jones, and J. Gormally, Eds.), Chap. 11, p. 271. Elsevier, New York, 1983. 24. Mao, P., and Thomas, J. K., Langmuir 9, 1504 (1993).
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